1. Field of the Invention
The present invention relates to a marine structure having superior corrosion resistance, and particularly to a marine structure having a titanium-clad steel plates being joined together in the many number of exposed joint ends.
2. Description of the Related Art
A marine structure usually involves legs made of square materials or steel pipes. The surface of each leg is treated with a corrosion resistant treatment, e.g., painting etc, to protect against sea water corrosion. This surface corrosion resistant treatment is not sufficient to protect the marine structure for years against splashing sea water. An area of the marine structure affected by the splashing sea water is called the splash zone and extends along the marine structure at sea water level.
To protect a marine structure at the splash zone from corrosion, as shown Japanese Unexamined Utility Model Publication No. 62-44948, the marine structure is coiled and covered with titanium-clad steel plates having superior corrosion resistance.
In the past, after the titanium-clad steel plates were disposed around the legs at the splash zone, ends of the titanium-clad steel plates were welded together.
But, since it was difficult to weld titanium to steel, various improved techniques have been proposed for this purpose. For example, Japanese Unexamined Patent Publication No. 2-52176 describes a recess formed on a steel at a boundary between a titanium layer and a carbon steel layer of each titanium-clad steel plate, and the recess is filed with an inert gas or a backing strip is inserted in the recess, in treating a butt welding, thereafter, the titanium layer and steel layer are individually welded with an uranami welding technique. Alternatively, Japanese Unexamined Patent Publication Nos. 2-280970 and 2-280969 disclose a technique of avoiding forming a joint of dissimilar metals.
FIGS. 5(a) and 5(b) in the present specification show examples of conventional methods of joining titanium-clad steel plates 1 together. Each of the plates 1 is made of a base layer 2 mainly composed of Fe and a titanium layer 3.
In FIG. 5(a), ends 4 of the base layers 2 of the titanium-clad steel plates 1 are welded together, and the titanium layers 3 are welded with a titanium spacer 6 to form titanium weld portions 7. This method forms a space 10 between the surface of the end 4 of the base layers 2 and the spacer 6, so that sea water may enter into the space 10 to corrode the base layers 2. In addition, the space 10 deteriorates the strength of the welded part. Further, the base layers 2 partly melt when the layers 3 and spacer 6 are welded together, and the molten metal is mixed with deposited metal of the weld portions 7 to produce a brittle layer of compounds such as TiC and TiN, or of intermetallic compounds such as Fe-Ti. This may cause cracks.
To prevent a formation of such a brittle layer, the method of FIG. 5(b) does not weld the spacer 6 with the titanium layers 3. Instead, the spacer 6 is only disposed between the titanium layers 3, and a patch plate 8 is placed over the spacer 6 and titanium layers 3. The patch plate 8 is joined with the titanium layers 3 by forming lap fillet weld portions 9. The patch plate 8 is made of titanium, similar to the titanium layers 3 and spacer 6, so that the brittle layers are not formed at the lap fillet weld portions 9.
This method, however, presents a complicated weld structure and forms a large space 10 when the patch plate 8 is welded to the titanium layers 3. It is hard to prevent sea water from entering the space 10. In addition, this method is inapplicable for a structure that is to be flexurally processed because the space 10 expands when the structure is bent.